If the 800xA system can run on Window Server 2008R2 or Window Server 2012
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5.1 need 32-bit operating system.
5.1 RevA/FP1 have separate 32 and 64 bit media bundles.
5.1 RevB/FP2 and onwards are 32/64 bit on same media.
Hence, 2008R2 can be used starting with 5.1 RevA/FP1.
With 2012 Server you must use 6.0.
3BUA000500 Third Party Software document contains the facts (hint: download ALL versions since the most recent will only cover the most recent xA revisions).
In addition to above points,
Windows Server 2012 R2 can be used for 800xa Systems for version 6.0
But if you are Planning for a new system with Virtualisation as scope, i suggest you to go for Windows Server 2012 R2 Datacenter.. In Datacenter version you can create any number of Virtual machines depending on your RAM..So if you are going for a newer system..You can go for Windows Server 2012 R2 Datacenter, where you can install 800xA directly on your machine as of now and in future if there are any plans for expansion, then in the same machine you can Create VM's using Hyper V.